Grishka - I don't know if you've ever used MuseScore. It's one of the standard (free) packages for music writing in Linux, but it is cross-platform for Windown, OSX and Linux. The latest version (1.3 ,it's not in Ubunut repositories yet, but there's a ppa for it) has an improved abc import plugin - it imported Adam Lay Ybounden (with middle=d added to V3/4) - without any problems. I'm not sure of the import mechanism yet, it said you needed to be connected to the internet to use it - but it may be a way forward. At the moment it doesn't (afaik) have an abc export facility, but that might be rectifiable. I'll have a look at the source code later and see what's involved. If an abc export was available, then we would have a proper graphical input method for creating abc files. You might still want to use eg abcm2ps for final printing (or even the Lilypond export) but it looks promising. It might also be nicer to have the abc import handle multitune files - it was confused by an abc stylesheet header on another file I tried to import and only imported the first tune from a multitune file - I think it probably stops at the first blank line. But these are relatively minor things to change.
If the import supported 2.1 and an export for 2.1 was written, it would be a possible uniform cross-platform program to use for abc